Yes, upvoting an answer that has already been downvoted cancels the downvote(0) and applies the upvote(+1). I know I've read Jeff talk about this behavior before... now, if I can only find where.
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heavydMar 3 '10 at 16:56

mmyers: Oh....I didn't know that...sigh...Of course to boost your rep whorin, I'll upvote and accept yer answer!.... :p That strangely is not mentioned about it or I have missed it...
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t0mm13bMar 3 '10 at 16:57

@mmyers: There done it! You should see a nice increase there on your rep! :P lol man...that should have been said somewhere obviously....
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t0mm13bMar 3 '10 at 16:58